April 10, 2012

At West Hunter, Gregory Cochran writes about the distinction between extremely deleterious genetic mutations that frequently kill people before they pass on their bad gene (e.g., Huntington's Disease) and mildly detrimental mutations that reduce Darwinian fitness in the range of 1 percent. Not surprisingly, the latter are more common because they can build up over the generations before they keep an individual from reproduciing.

... So… most genetic load in humans is made up of many, many mutations that each have fairly small effects. A smaller fraction of the genetic load consists of mutations with big effects on fitness.

... One important point is that a single highly deleterious mutation has a good chance of pushing the whole organism in some odd direction in phenotype space. In other words, the same mutation that drops your IQ, or damages your heart, may also make you look funny. At lower IQs, more and more kids are considered to suffer from ‘organic’ retardation. On the other hand, a higher-than-average number of small-effect mutations should also interfere with really complex systems such as the brain (and reduce IQ), but because of the law of large numbers, wouldn’t tend to have any particular direction in phenotype space. As far as I can tell, an extra-large dose of small-effect mutations, which we will henceforth call genetic noise, would not make you funny-looking.

Would the converse be true? Would good-looking but not very bright people also tend to have more genetic noise, as well, just in different places

Individuals can vary in the amount of genetic noise they carry, and populations can as well, depending on the relative intensity of selection and on the mutation rate, which might also differ. For example, although having an unusually old father does not much affect the amount of genetic noise an individual carries, a culture in which fathers were typically 55 would undoubtedly accumulate an unusually high amount of genetic noise, over a couple of millennia.

If a kid’s parents have a higher-than-average amount of genetic noise, on average the kid will as well. This sure looks like what we usually call non-organic or familial retardation.

Most of the within-population variation in IQ looks to be familial rather than organic. If I’m right, this means that most IQ variation – what we might call the normal range – is caused by differences in the number of slightly deleterious mutations. None of them would show up in a QTL search, because all are rare. And that is where we stand thus far: no intelligence QTLs have been found – although you never know what you’ll see in the next population. On the other hand, shared chromosomal segments would mostly contain the same slightly deleterious mutations, and so IQ should correlate with genetic similarity, which is what Visscher has found.

So, think of models for the genetics of IQ like horsepower in cars. In one model, a lot of people get the engine designed for 200 horsepower, some get the engine designed for 400 horsepower, and some get the engine designed for 100 horsepower. Occasionally, something very bad happens in the manufacturing process or the maintenance process (e.g., Down's Syndrome) and people get an engine that only delivers 50 horsepower.

Cochran's new model is at the other end of the spectrum: Most people get engines designed for 300 horsepower, but there are a whole lot of minor glitches in the manufacturing process (some because the blueprints have had accumulating errors creep into them in copying until they get thrown out, some de novo). So, most people get a mental engine somewhere in the 100 to 300 horsepower range, typically falling out in a bell curve.

But what about the 400 horsepower people known to history?

Many great scientists and mathematicians have likely had relatively low levels of genetic noise combined with some fairly deleterious de novo mutations; with the net effect of a powerful mental engine strangely focused on some particular topic not directly related to fitness. Low noise, high weirdness. Math, not sheilas. One might look for advanced paternal age in such cases.

Read the whole thing there. There's one phrase in it that hints at the next stage of his theory, but I'll leave it at that.

I wonder how you'd go about removing this genetic noise. Averaging pictures leads to an amazingly pretty face, because it has no defects. Might averaging across a million people do better than trying to figure out what makes geniuses similar to each other?

"Averaging pictures leads to an amazingly pretty face, because it has no defects. Might averaging across a million people do better than trying to figure out what makes geniuses similar to each other?"

So does this mean that back 30,000 years ago before man had accumulated all these mutations everyone had an IQ of ~140? Seems very unlikely, but that's what you get if you take this theory to its logical conclusion.

So does this mean that back 30,000 years ago before man had accumulated all these mutations everyone had an IQ of ~140? Seems very unlikely, but that's what you get if you take this theory to its logical conclusion.

I'm 47 yo white guy. Average IQ. Wife 35 yo. Smarter. Not sure that I want to spend the last 30 years of my life taking care of a special needs kid. First cousin had autistic kid in her 40's. Brother had a kid with a dusting of autism in his mid 40's. Should we have a kid?

"On the other hand, shared chromosomal segments would mostly contain the same slightly deleterious mutations, and so IQ should correlate with genetic similarity, which is what Visscher has found."

The cost of outbreeding. We are always being told that outbreeding increases fitness. But Cochran is saying that it might increase noise. Therefore, you would expect the 400 hp's to come from homogenous (less noisy) groups. You should also to see some small average fall off (less intelligent than their sum should be) when an Englishman marries a Russian because they are each bringing different glitches to the table. Interesting.

"Many great scientists and mathematicians have likely had relatively low levels of genetic noise combined with some fairly deleterious de novo mutations; with the net effect of a powerful mental engine strangely focused on some particular topic not directly related to fitness. Low noise, high weirdness. Math, not sheilas. One might look for advanced paternal age in such cases."

A couple of observations in connection with this:

1) Melancholia (what we would now call bipolar syndrome) was anciently associated with creative genius. See F. Saxl, et al., "Saturn and Melancholy."

2) Aristotle held - and his opinion was apparently shared by many in classical antiquity - that the ideal ages for marriage were 18 in the case of the female, and 35 in that of the male.

The late, great WD Hamilton once wrote an essay entitled 'The Planetary Hospital' - a dystopian account of a future plagued by genomic degradation, due to 'natural selection' being abandoned. I've tried hard to find this on the web, but I search in vain.

Well, if indeed geniuses should tend under this theory to have older fathers, is that what the empirical evidence shows?

That's not what he's saying. You can't be a top salesman, or a CEO, or a senator if you've got some weirdness in you do to one of these highly deleterious mutations. But you can be a top-echelon mathematician, and sure enough such top-notch mathematicians have existed. So among these weird super-geniuses (not all super-geniuses), we might find older fathers to be common.

The key is he said "many" not "most". The weirdness is not necessary to a genius career, it just doesn't hurt that much.

Why can't a mutation increase your fitness as opposed to decrease it? This article seems to associate mutations only with negative Darwinian effects.

It seems to me that a collection of "small-effect" mutations could explain why two average-IQ parents are able to have the occasional offspring who is a little above average. On average, maybe these mutations would not have a large affect in either direction - but maybe sometimes you'll maybe see a 5-10% change in IQ (this is just a fictional number I created) in either direction.

Statistically, its seems improbable that those "small-effect" mutations would push you very much in a particular direction beyond just small statistical quirks. However some kind of eugenic selection could result in, over time, the IQ-increasing mutations being selected. Maybe this explains people who are above-average in IQ but not at the level of "super genius"? Maybe it takes the actual "large-effect" mutations to get up to that level?

I also wonder about highly-inbred and highly-selected populations like Ashkenazi Jews. It is theorized that intelligence has been selected for this group (due to historical circumstances). What about the effects of inbreeding? How does that play a role in the expression of mutations? I know A. Jews have a statistically higher than normal incidence of various genetic disorders - would that play the same role for "large-effect" types of mutations in IQ/genius?

(Disclosure - I'm not well-educated in genetics or biology. Just tring to infer and extrapolate information from the article and bits of knowledge I've picked up reading HBD blogs.)

From Frost, "Greg thinks there may be infectious organisms that originally developed from Neanderthal tumors several tens of millennia ago. These organisms might look like amoebae, but genetically they would be Neanderthals."http://evoandproud.blogspot.co.uk/2008/12/neanderthals-in-my-sinus.html

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